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MathGroup Archive 1994

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Listable

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Listable
  • From: toet at sunwise.tm.tno.nl (Lex Toet)
  • Date: Thu, 28 Apr 94 10:05:47 +0200

To MathGroup:

the Mma book states that Listable is equivalent to Thread[f[args]] (p. 274)

I want to apply a function to corresponding elements of several 2D matrices of 
identical dimensions.

I found that making the function Listable does the job, just like MapThread
with level 2.

I do not understand this, since Listable should apply the function at level 1,
not level 2.

In[1]:= m1={{a,b},{c,d}};

In[2]:= m2={{A,B},{C,D}};

In[3]:= Thread[f[m1,m2]]

Out[3]= {f[{a, b}, {A, B}], f[{c, d}, {C, D}]}    <- applies f at level 1, like expected

In[4]:= MapThread[f,{m1,m2},2]

Out[4]= {{f[a, A], f[b, B]}, {f[c, C], f[d, D]}}  <- applies f at level 2

In[5]:= SetAttributes[f,Listable]

In[6]:= f[m1,m2]

Out[6]= {{f[a, A], f[b, B]}, {f[c, C], f[d, D]}}  <- applies f at level 2 !?


So how can Listable be equivalent to Thread ???????





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